This invention relates to an improved optical glass having special application as a segment glass (or button) for use in the manufacture of compound optical elements such as multifocal corrective lenses.
Multifocal ophthalmic lenses are conventionally fabricated by fusing one or more segments of a glass with a bulk crown glass. The refractive index of the segment glass is higher than that of the bulk glass making up the lens blank. The index of refraction of the segment glass determines the degree of visual correction which is achieved.
In addition to the necessary index of refraction, segment glasses must closely match certain critical properties of the base glass. These include comparable values of thermal expansion, viscosity characteristics, and softening point. Furthermore, it is necessary that the segment glass have a relatively low dispersion and be of overall high quality. Of course, it is also preferred that the segment glass be of low density, thereby providing lower weight pieces.
Many such segment glasses are known and are matched with various types of crown glass. See, for example, U.S. Pat. No. 4,351,906. However, the segment glass of this reference and others is inappropriate for a prime use of this invention, i.e., as a segment glass in conjunction with Schott Glass S-1018. This is clear from the dramatically different composition which is involved.
Other prior art references disclosing somewhat related glasses are U.S. Pat. No. 3,022,182, JP-PS No. 7573914 and JP-PS No. 7828448. The first of these is significantly different, e.g., in that it permits no boron or aluminum components. The second is different, e.g., in that it requires significant amounts of tantalum and zirconium. The third is different in that it requires significant amounts of lithium, for example.